1. Technical Field
Embodiments relate to components of protection against overvoltages, and more specifically to a bidirectional Shockley diode.
Two main types of bidirectional Shockley diodes can be distinguished according to the technology used for their manufacturing: planar diodes and mesa diodes.
2. Discussion of the Related Art
FIG. 1 shows an example of a planar-type bidirectional Shockley diode. This device is formed in a lightly-doped N-type substrate 1 (with typically from 1014 to 1015 atoms/cm3). A P-type well 3 is formed on the upper surface side and a P-type well 5 is formed on the lower surface side. Usually, these wells are symmetrical and of same doping.
An N-type region 7 is formed in upper well 3 and an N-type region 9 is formed in lower well 5, regions 7 and 9 being heavily doped. Regions 7 and 9 are complementary in projection and substantially of the same surface area. As shown, regions 7 and 9 are generally interrupted by emitter short-circuits.
The component periphery, between the well limit and the chip edge, is coated with an insulating layer, respectively 11 at its upper surface and 13 at its lower surface. Also at the periphery of the structure, at the edge of the chip, heavily-doped N-type rings, respectively 15 at the upper surface and 16 at the lower surface, are used as a channel stop.
The upper surface is coated with a metallization A1 and the lower surface is coated with a metallization A2. When a positive voltage is applied on terminal A1, PNPN Shockley diode 3-1-5-9 is likely to turn on, when the breakdown voltage of the reverse junction between regions 1 and 5 is exceeded. When a positive voltage is applied to terminal A2, PNPN Shockley diode 5-1-3-7 is likely to turn on, when the breakdown voltage of the reverse junction between regions 1 and 3 is exceeded.
To obtain breakdown voltages that are independent of from the substrate doping and to accurately define the volume of the breakdown areas, an N region 17 is arranged in front of upper N region 7 at the interface between well 3 and substrate 1 and an N region 19 is arranged in front of lower N region 9 at the interface between well 5 and substrate 1. N regions 17 and 19 will be called buried regions and for example result from implantations performed before the forming of P wells 3 and 5. Thus, junction J2 between N region 17 and P well 3 and junction J1 between P well 5 and N region 19 determine the breakdown voltages of the device. Optional P-type buried regions 21 and 23 have further been shown in front of N-type buried regions 19 and 17, respectively. Buried regions 21 and 23 aim at decreasing the effective thickness of substrate 1 in each of the Shockley diodes, to decrease the on-state resistance of the protection device.
A bidirectional Shockley diode of planar type such as shown in FIG. 1 provides satisfactory results. However, in many cases, it is preferred for technological reasons to form mesa-type diodes, especially because it is much simpler to form relatively deep P regions (more than 30 μm for diodes adapted to breakdown voltages ranging from 50 to 400 V) with no masking.
FIG. 2 shows an example of a mesa-type bidirectional Shockley diode. To simplify the description, layers similar to those in FIG. 1 have been designated with the same reference numerals. An essential difference is that, instead of forming local P-type wells 3 and 5 on either side of the substrate, uniform P-type layers, also designated with reference numerals 3 and 5, are formed with no masking on both surfaces of the substrate. The diode is delimited by peripheral grooves, respectively 31 on the upper surface side and 33 on the lower surface side, filled with an appropriate insulating material, respectively 35 and 37, currently a glassivation. Grooves cut the junctions between the substrate and P layers 3, 5.
Generally, as compared with a planar-type diode, a mesa-type bidirectional Shockley diode, biased to a voltage smaller than its breakdown voltage, has greater leakage currents. Further, the leakage currents tend to increase during the lifetime of the component when it is submitted to external stress, such as a lengthy biasing and a high temperature. In FIG. 2, equipotential line VA2 when a positive potential difference (VA2-VA1) is applied between electrodes A2 and A1 has been illustrated in bold dotted lines. The technology used to create the mesa groove, its specific geometric shape, and the nature of the passivating materials explain the distribution of the equipotential lines at the edges of the component, as well as their variation in the presence of stress. The electric field thus present at the passivation-silicon interfaces is responsible for the high leakage currents.
Many solutions and mesa-type bidirectional Shockley diode structures have been provided to overcome these disadvantages. However, some known solutions are relatively complex and require either a significant increase in the surface area of the bidirectional Shockley diode, or additional manufacturing steps with respect to those required for the manufacturing of a component such as that illustrated in FIG. 2.
There thus is a need for a simple mesa-type bidirectional Shockley diode with a low leakage current that remains steady along time.